(1) Field of the Invention
The present invention relates to a valve which is used in a nuclear power plant and a thermal power plant, and is provided with a valve seat (a part of a slide or contact portion), and more particularly to a reforming of a metal metallographic structure and a technique of improving a material property going with the same, with regard to a material of a weld overlay used in a valve seat.
(2) Description of Related Art
In general, in an equipment such as a valve or the like having a valve seat, which is used in a power generation plant or the like, it is requested for the valve seat to stand up to an impact caused by opening and closing the valve during an actual operation, for the valve seat to stand up to a high flow rate, a cavitation and an impact of a liquid drop, and for the valve seat to have an excellent corrosion resistance under a high-temperature and high-pressure environment.
In this regard, a valve provided with a valve seat of a hard material of weld overlay has been conventionally prepared by using a method of dissolving a Co base alloy, an Ni base alloy or an Fe base alloy which is excellent in a corrosion resistance and an abrasion resistance at a high temperature, and weld overlaying on a valve case and a valve body.
However, in the case of weld overlaying the surface deposit metal by the high temperature dissolving, a metallographic structure of the weld overlay valve seat such as the Co base alloy, the Ni base alloy, the Fe base alloy or the like which is used generally, takes on the same structure aspect as a metallographic structure of a metal material which is prepared by casting, a dendrite (a base portion) crystallizes at a time of cooling from a molten state at the high temperature dissolving time to a solid state, and a chemical compound phase such as a dendrite like eutectic carbide or a boride is formed in a gap of the dendrite.
In the surface deposit metal such as the Co base alloy, the Ni base alloy, the Fe base alloy or the like, a corrosion resistance and an abrasion resistance of a chemical compound phase such a dendrite like eutectic carbide crystallizing to the dendrite gap or the like is lower in comparison with the dendrite base portion, it is impossible to stop a progress of a selective corrosion of the chemical compound phase caused by a fluid coming into contact with the surface deposit metal and a surface erosion of the surface deposit metal generated by an erosion or the like, in the light of its characteristic reason, and it is necessary to carry out a frequent inspecting and repairing work.
On the other hand, for the purpose of suppressing or inhibiting the selective corrosion of the chemical compound phase such as the dendrite like eutectic carbide or the like, for example, as shown in patent document 1 (JP-A-2000-273573), there has been invented a corrosion resisting and abrasion resisting alloy of Co base alloy, Ni base alloy or Fe base alloy in which a chemical compound phase such as an eutectic carbide or the like is dispersed as a blocky shape or a granular shape. Further, for example, as shown in patent document 2 (JP-A-2000-095219), a diffusion bonding technique has been invented as a technique of preparing a valve seat by using the corrosion resisting and abrasion resisting alloy, and bonding to the valve case or the valve body while the valve seat keeps a metallographic structure having a blocky or granular chemical compound phase. However, since the diffusion bonding method is a technique of inserting an insert material which is different in a mechanical characteristic and a composition from the valve seat, between the valve case or the valve body and the valve seat, and bonding in accordance with a heat treatment, there is fear of a reduction of the mechanical characteristic in an intermediate layer formed between the valve case or the valve body and the valve seat, in some materials, as well as an increase of a man hour.
If the valve provided with the valve seat of the hard surface deposit metal is prepared by dissolving the surface deposit metal such as the Co base alloy, the Ni base alloy, the Fe base alloy or the like at a high temperature and buildup welding to the valve case and the valve body, like the conventional manner, the metallographic structure of the valve seat necessarily comes to a dendrite structure, and the chemical compound phase such as the eutectic carbide or the like is formed like a mesh in the dendrite gap. Since the chemical compound phase such as the eutectic carbide or the like is continuously distributed in the dendrite gap, once the selective corrosion of the chemical compound phase such as the eutectic carbide or the like is generated, the corrosion continuously makes progress, and a surface roughness of the valve seat and a reduction of a leakage resistance going therewith are generated.